INNER JOIN usage and examples

Overview

INNER JOIN returns only row combinations that satisfy the ON condition on both sides; unmatched rows are excluded. If a join key is not unique, one row can match several rows and multiply the result. Check key uniqueness and row counts before adding aggregation or further filters.

Basic syntax

SELECT o.id, c.name FROM orders o INNER JOIN customers c ON o.customer_id = c.id;

Examples

INNER JOIN example

Join orders to customers by customer_id and return only orders that have a matching customer.

Sample table

Table：orders

orders
id	customer_id
1	101
2	102
3	101
4	104

Table：customers

customers
id	name
101	Alice
102	Bob
103	Cindy

Run result

result
id	name
1	Alice
2	Bob
3	Alice

Common issues

Which rows does INNER JOIN return?

It returns only rows that match on both sides according to the ON condition.

Why does JOIN increase row counts?

The join key may have duplicates on one or both sides, creating one-to-many or many-to-many matches.

Can ON use multiple columns?

Yes. Use AND to combine multiple join conditions.

Keep reading

LEFT JOIN CROSS JOIN SELF JOIN FULL OUTER JOIN

INNER JOIN